The present invention relates to bromine free catalyst system, which exhibits high activity and selectivity in the oxidation of alcohols to aldehydes or acids using NaOCl as an oxidant. More specifically, the invention relates to a catalyst system, which comprises a synergistic couple of 2,2,6,6,-tetramethylpiperidinyloxy catalyst (hereinafter referred to as xe2x80x9cTEMPOxe2x80x9d or xe2x80x9cTEMPO catalystxe2x80x9d) and Na2B4O7 co-catalyst (CC), which is more active and shows higher selectivity than the known TEMPOxe2x80x94NaBr system. Such a synergistic couple is particularly useful for, but not limited to, oxidation of primary aliphatic and aromatic alcohols. The system also permits carrying out the oxidation without the need for solvents.
The catalytic oxidation of alcohols selectively to carbonyl compounds is probably one of the most important transformations in the synthetic organic chemistry. A large number of oxidants have been reported in the literature and most of them are based on transition metal oxides such as chromium and manganese (S. Kirk-Othmer Mitchell, Enciclopedia of Chemical Technology, 4th ed., Wiley-Interscienc, New York, Vol.2, p 481, (1992); Hudlicky, M. xe2x80x9cOxidations in Organic Chemistryxe2x80x9d, ACS Monograph No.186 American Chemical Society Washington D.C. (1990); Sheldon R. A., Kochi J. K. Metal Catalized Oxidation of Organic Compounds. New York, Academic Press, 1981; Ley, S. V., Madin, A,. In comprehensive Organic Synthesis, Trost B., Fleming, I., Eds.; Pergamon Oxford, 1991; Vol 7, p251; Mijs. W. J., DeJonge, C.R.H.I. Organic Synthesis by Oxidation with Metal Compounds ; Plenum: New York, 1968). Since most of the oxidants and the products of their transformation are toxic species, their use creates serious problems concerning their handling and disposal. A serious drawback to the use of these reagents, for both cost and toxicity reasons, is the need to use them in large excess over the required reaction stoicheometry. The search for efficient, easily accessible catalysts and xe2x80x9ccleanxe2x80x9d oxidants such as hydrogen peroxide, hydroperoxides or molecular oxygen for industrial applications is still a challenge (Dijksman, A., Arends I.W.C.E. and Sheldon R., Chem. Commun., 1999, 1591-1592; Marko I. E., P. R. Giles, Tsukazaki M., Brown S. M. and Urch C. J., Science, 19696, 274, 2044). A large number of transition metal complexes and oxidants have been reported to catalyze the selective oxidation of primary alcohols to aldehydes with varying levels of effectiveness such as RuCl3-NaBrO3 (Konemoto S., Tomoioka S., Oshima K.), Bull. Chem. Soc. Japan. 1986. V.59. N1, P.105), Bu4NRuO4-4-Methylmorpholine N-oxide (Griffith W. P., Ley S. V., Whitcombe G. P., White A. D)., Chem. Commun. 1987, N21, p.1625), H2O2 and tert-Butylhydroperoxide (t-BuOOH) (Y. Tsuji, T. Ohta, T. Ido et al.), J. Organometalic Chemistry, 270, 333 (1984), (T. M. Jiang, J. C. Hwang, H. O. Ho, C. Y. Chen). J. Chin. Chem. Soc., 35, 135, (1988). The methods described have only limited use since the overall yields are low and some of them require the application of precious metal complexes or expensive primary oxidants.
A particularly convenient procedure for the oxidation of primary and secondary alcohols is reported by Anelli and co-workers (J. Organic Chemistry, 1987, 52, 2559; J. Organic Chemistry, 1989, 54, 2970). The oxidation has been carried out in a two-phase system (CH2Cl2-water) utilizing the TEMPO as a catalyst and cheap and readily accessible NaOCl as an oxidant. The co-catalyst KBr enhances the reaction rate and the aqueous phase is buffered at pH 8.5-9.5 using NaHCO3. The use of a quaternary ammonium salt as a phase transfer catalyst furthers the oxidation of alcohols to carboxylic acids. The same procedure was modified by using NaClO2 as the oxidant in the presence of catalytic amounts of TEMPO and NaOCl. This led to the formation of the carboxylic acid as the main product (U.S. Pat. No. 6,127,573).
Prakash et al. in U.S. Pat. No. 5,856,584 report a similar procedure for oxidation of 3,3-Dimethyl-1-butanol. According to this procedure the 3,3-Dimethyl-1-butanol is oxidized to 3,3-Dimethylbutyraldehyde with NaOCl in a two-phase system using CH2Cl2 as a reaction solvent. The stable 2,2,6,6-tetramethyl-1-piperidinyloxy free radical and KBr are used as an efficient catalytic system to produce the desired aldehyde in 80% isolated yield.
In another development of the TEMPO mediated oxidation of primary alcohols to aldehydes, Sheldon and co-workers have used a polymeric version of the TEMPO catalyst PIPO, obtained by oxidation of the commercially available poly[(6-[1,1,3,3-tetramethylbutyl)amino]-s-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidyl)imino] hexamethylene[(2,2,6,6-tetramethyl-4-piperidinyl)imino]), known also as Chimasorb 944, (in Chem.Commun., 2000, 271-272). The procedure produces aldehydes in high yields in solvent free conditions only for aromatic alcohols. The oxidation of primary alcohols leads to significant over-oxidation to carboxylic acid.
U.S. Pat. No. 5,821,374 describes the use of N-chloro compounds such as N-chloro-4-toluenesulfonamide sodium salt (Chloramine T) or N-chloro-benzene sulfonamide sodium salt (Chloramine B) as an oxidant in the TEMPO catalyzed oxidation of primary alcohols to aldehydes. The major drawback to this method is the use of large amounts of solvents and the toxicity of the N-chlorinated aromatics used as oxidants.
U.S. Pat. No. 6,335,464 describes a polymer supported TEMPO catalyst which, combined with a NaBr co-catalyst, was used to electrocatalytically oxidize primary alcohols to carboxylic acids using NaOCl as the oxidant. There is no report on the use of the procedure for selectively forming the corresponding aldehydes and the possibility for re-use of the catalyst.
Despite the extensive work reported in the area of the selective oxidation of primary alcohols there is still a continuous need for developing highly efficient and economical oxidation methods which do not require the use of organic solvents, can be carried out with environmentally friendly oxidants and do not require the use of bromine based co-catalysts. It is the object of the present invention to provide such an oxidation method.
The process according to this invention comprises oxidizing primary or secondary alcohols with an oxidant in the presence of a catalyst of formula II or III and a co-catalyst. 
In Formulas (II) and (III), R1, R2, R3 and R4 independently are lower alkyl or substituted alkyl groups of the same or different structures. R5 and R4 are hydrogen, alkyl or lower alkoxy or one is hydrogen and the other is lower alkoxy, hydroxy, amino, alkyl or dialkylamino, alkylcarbonyloxy, alkylcarbonylamino, or R5 and R6 are ketal. The Yxe2x88x92 group is an anion.
The co-catalyst according to the invention is a polyoxy anion or metal salt having formula IV, where M1 is a metal ion from Group IA or IIA and M2 is an ion from Group IIIa, IVa, IVB, Va, VB, VIA, VIB, VIIB or VIII of the Periodic Table of Elements.
The TEMPO/co-catalyst promoted oxidation is described by the following reaction shown in Scheme 1. According to this, the oxidation takes place via a cascade mechanism in which a number of oxidizing species exist in a dynamic equilibrium. Formally, the hypochlorite anion oxidizes the co-catalyst (CC) to its oxidized form (CCO*), which in turn transfers the chain over to TEMPO oxoammonium salt. The xe2x80x9coxidizedxe2x80x9d form of TEMPO converts the primary alcohol to aldehyde in the last redox cycle. The reaction takes place at the pH of the bleach solution in the range of 8.6-9.5 in which the hypochlorite anion is relatively stable and at the same time the rate within the CIOxe2x88x92/Clxe2x88x92-CC/CCO* is high enough to sustain high overall rates of alcohol oxidation. To maintain the desired pH, a NaHCO3 or K2CO3 could be effectively used. Since the desired aldehyde under the current oxidizing conditions can easily undergo further conversion to the corresponding acid derivative, the reaction temperature is kept at or below 0xc2x0 C.